1. Field of the Invention
The present invention relates to a laminate superior in formability, especially drawability, and gas barrier properties.
2. Description of the Prior Art
Among many thermoplastic resins capable of melt forming, ethylene-vinyl alcohol copolymer (referred to as EVOH hereinafter) is noted for its gas barrier properties, oil resistance, and solvent resistance. It has found use as film, sheet, containers, etc. in the area of packaging, especially food packaging.
EVOH resin, however, has some disadvantages. As compared with other thermoplastic resins such as polyolefins, it is poor in formability, especially drawability, and it easily suffers cracking and uneven stretching when it undergoes plastic working such as deep draw, air-pressure forming, plug-assist forming, and stretch blow molding. In addition, it is liable to exhibit uneven clarity after forming.
To meet the recent requirements for sophisticated performance of packaging materials, an EVOH resin is often used in combination with other resins such as polyethylene, polypropylene, and polystyrene in the form of a laminate. In this case, an EVOH resin constitutes an interlayer of a multi-layer laminate and imparts improved physical properties to the laminate. The laminate is required to be capable of being pressure formed at a draw ratio of 0.5 to 3. This requirement, however, is not satisfied with an EVOH resin containing ethylene in the range of 25 to 45 mol%, especially 25 to 40 mol%, which is the best range for obtaining good gas barrier properties. The requirement is satisfied only at a sacrifice of the gas barrier properties with an EVOH resin containing more than 40 mol%, especially more than 45 mol% of ethylene.
EVOH film also has disadvantages when it is stretched in the form of a single film or a laminated film with other thermoplastic resins. In the case of EVOH film containing ethylene in the desired range of 25 to 40 mol%, it cannot be biaxially oriented satisfactorily unless it is properly moisturized. (See Japanese Patent Laid-open Nos. 14476/1975, 15570/1977, and 30670/1978.) Where EVOH is used as an interlayer of a multi-layer laminate or as a layer of a double-layer laminate, the EVOH layer easily suffers cracking and uneven stretching, as in the case of deep draw and stretch blow molding, if it is oriented, especially biaxially, in a substantially water-free state.
On the other hand, several methods have been proposed for improving the melt forming performance of EVOH. According to Japanese Patent Laid-open No. 20976/1983, two kinds of EVOH resins which differ in ethylene content and composition are compounded together and the resulting compound is formed into films, sheets, and containers. According to Japanese Patent Laid-open Nos. 129035/1983 and 154755/1983, EVOH is blended with nylon. In the former case, the melt forming performance is improved to some extent but the resulting formed parts are not satisfactory with respect to clarity. In the latter case, blending with nylon, which is poor in gas barrier properties, impairs EVOH's good gas barrier properties, and the resulting blend is poor in thermal stability and forms gels at the time of melt forming. (Presumably, the gels are reaction products of EVOH and nylon.)
In order to provide a food packaging material superior in formability, there was proposed in Japanese Patent Laid-open No. 8125/1982 an EVOH resin modified with an .alpha.-olefin represented by propylene. The modified resin, however, is not yet put to practical use because of insufficient gas barrier properties, although there is some improvement in formability and clarity of formed parts.